1. Field of the Invention
The invention refers to a process of cooling a material bed of bulk material such as cement clinker on grate coolers, with a cooling medium such as air being passed through the material bed in an upward direction, and also to a decelerating device for carrying out such a cooling process.
2. Description of Prior Art
In cement production as well as in other rotary kiln processes for burning lime, magnesite, iron ore, phosphate or the like, the material is fed to a cooler upon reaching its process temperature for direct preheating of the combustion air. Attainable degrees of heat recuperation by means of the combustion air depend on the mass flow rates, specific calorific values, and the efficiency of the heat exchange.
In modern cement plants, due to fuel savings, only 0.8 to 0.9 nm.sup.3 of combustion air per kg of cement clinker are available for cooling. Yet the recuperation could theoretically still be 100%. In practice, however, most cooler types attain only about 70% recuperation. With grate coolers the residual heat may be wasted with great quantities of off-gas. Occasionally it is partially used indirectly for the cement process. An increase of 10% the recuperation would effect a heat saving of 5% in the cement process.
The recuperation is mainly limited by the uneven size distribution of the cement clinker as well as by the different efficiencies of heat exchange of the different cooler types for the coarse and the fine clinker fractions, respectively.
The rotary kiln and satellite cooler types apply the counter flow principle. They are well suited for the fines, which are evenly exposed to the air stream in screens. The air velocities permissible for the fines are too small to sufficiently cool the coarse fraction, which in addition is passing rotary kilns faster than fines. Furthermore, coarse chunks disturb the even air distribution to the satellite tubes.
A known shaft kiln--see e.g. DE-PS No. 15 58 609--also applies the counter flow principle and cools the coarse material sufficiently when the air flow and retention time are chosen appropriately. However, the fines collect an concentrate in a fluidized bed or in so-called dead corners and are discharged irregularly upon the collapse of the air flow and consequently not cooled sufficiently.
In grate coolers the cross flow principle is applied sacrificing the counter flow for better air distribution to the coarse and the fine material. The cross flow may pass the bed one or more times in counter flow sense. The improved air distribution is achieved by increasing the surface of the grate in connection with thin beds of less than 0.5 m, and air flows forced towards each grate section by partition walls forming separate air compartments beneath the grate. Following observation of the air distribution during operation single grate plates can be made impermeable or motionless to even out air and material distribution. Thus, in cross flow grates the coarse and the fine material are cooled in a like manner, and in practice one may therefore attain the same degree of recuperation by means of counter flow coolers.
Attempts were made to increase the bed depth with the same air velocity by higher pressure increasing the retention time, but they resulted in less recuperation, which can be explained by worse air distribution and local fines collection effected by less stirring of the bed by less grate action.
Other attempts were made increasing the air velocity, which resulted in uncontrollable movements of the fines (red river. Thus, with uneven size distributions, bed depths are not above 0.5 m and air velocities are kept below fluidizing the fines.
As a consequence of the different suitability of the coolers for the coarse and the fine material, solutions were proposed providing a separation of the coarse and the fine material and their separate cooling.
It is known (see DE-AS No. 23 43 339) to separate the fines in the hot feeder zone of a grate cooler by high air velocities and--possibly aided by laterally sloping grates--to discharge the fines to the sides. Thus, it is possible to cool the coarse material on the grate and the fines in additional transverse fluidizing coolers.
According to another known process (DE-AS No. 23 07 165) the separation of the coarse and the fine material is performed at an upper cooling level formed by a grid or screen carrying the coarse fraction, while the fines are cooled by the same air flow in a fluidized bed thereunder.
Neither of these two proposals are known to have been realized. This may be because of the substantial requirement of equipment in connection with the unpredictability of the size distribution, which is familiar to someone skilled in the art, and which makes the size of the separate cooling systems unpredictable as well.
3. Objects of the Invention
It is therefore an object of the invention to improve the heat recuperation of grate coolers.
In this connection, it is another object of the invention to increase the bed depth, thus providing a longer passage of the combustion air to be preheated in order to increase the temperature but without discharging the still hot upper layers of the material bed at the same time as the well cooled bottom layers.
It is yet another object of the invention to improve the heat recuperation of grate coolers increasing the air velocity, and to control the movement of the fine fraction, which is uncontrolled according to present observations, in order to obtain suitable retention times of the coarse and the fine material respectively in the same cooler.
According to a still further object of the invention it is designed to recuperate the heat from the hot fines, generally passing through the grate.